1. Field of the Invention
The present invention relates generally to a suspension system of a vehicle such as an automobile, and more particularly, to an improvement of a double wishbone type suspension system of a rear wheel of an automobile so as to operate with discrimination of road irregularities and braking of the wheel.
2. Description of the Prior Art
As a rear wheel suspension system of automobiles there is known a modified double wishbone type suspension system having an external appearance such as somewhat diagrammatically shown in FIGS. 1 and 3 of the present application in plan and perspective views, respectively, though these figures show indeed an embodiment of the present invention which is embodied in certain details of the illustrated construction for an improvement of the wheel alignment performance of such a suspension system, as will be appreciated hereinbelow. In these figures, 10 is an axle hub for supporting a rear right wheel 12 of an automobile, and is supported from a vehicle frame 22 by a lower arm 18 and an upper arm 16.
As will be noted from the figures, in such a suspension construction, the inboard end of a rear leg 18b of the generally A-shaped lower arm 18 is shifted substantially inboard relative to the inboard end of a front leg 18a of the lower arm 18, thereby defining a common inboard pivot axis 28 connecting the centers of the front and rear inboard ends of the lower arm 18 substantially inclined with respect to the longitudinal direction of the vehicle, such as to define an angle .theta.. The rear suspension of this type is advantageous in providing a compact rear frame construction of an automobile having a flush back body style. However, although the inboard end of the front leg 18a is pivotably connected with the vehicle body 22 via a pivot joint 26 having a pivot axis coincident with the common pivot axis 28, it is considered to arrange a pivot joint 30 for pivotably mounting the inboard end of the rear leg 18b to have a pivot axis 32 oriented in the longitudinal direction of the vehicle in order to ensure the longitudinal alignment of the wheel 12 during as much bounding and rebounding movement as possible around a neutral position thereof. Therefore, in order for such a suspension system to be operable to allow the bounding and rebounding movement of the wheel, the pivot joints 26 and 30 must each incorporate an elastic bush which allows certain three dimensional movements between the members joined thereby.
As a result, as will be seen in FIG. 2, when the axle hub 10 is applied with a backward force F due to irregularities of road surface or braking of the wheel 12, the front inboard end of the lower arm 18 supported by the pivot joint 26 is shifted along the common pivot axis 28, while the rear inboard end of the lower arm 18 supported by the pivot joint 30 is shifted along the pivot axis 32, so that the lower arm 18, and the axle hub 10 as well, is turned counter-clockwise around a point O3 defined as a point of intersection of a straight line L1 extended from the center O1 of the pivot joint 26 to be perpendicular to the pivot axis 28 and a straight line L2 extended from the center O2 of the pivot joint 30 to be perpendicular to the pivot axis 32, thereby causing a toeing in of the wheel 12.
A toeing in of the rear wheels during a braking of the vehicle is desirable, in order to compensate for a decrease of the load on the rear wheels during the braking due to a forward shifting of the load balance on the four wheels, while, since a pair of right and left wheels will be toed in generally for a same absolute angle due to the braking, the running stability of the vehicle will not be damaged. However, a toeing in of the wheel according to the above-mentioned mechanism, when caused by irregularities of road surface, will often occur at only one of the right and left wheels, getting undesirable for ensuring the running stability of the vehicle, while providing no such advantage as available in the case of braking.